Increasing RO recovery is not just a case of adding stages!

I had an engineer recently tell me he was working on a project where they needed to minimize RO brine volume and he wanted at least 90% recovery, so please quote a 3-stage system… Antiscalant projections on the raw water quality showed 78% recovery at best, even with acid dosing. The chemistry won’t allow any higher recovery on this water on matter how many stages the RO system has, and don’t get me started on whether CCRO can do better (I’ll discuss this later).

That is the fourth inquiry like this I have had in the past 6 months and the second from this engineer who is finding it hard to understand the limitations on recovery… so I thought I would put the explanation in print here.

Before you start to think of how many stages you need to design the RO system for, you first need to look at the water chemistry to see what recovery is possible. All the antiscalant manufacturers have projection programs where the raw water quality data is entered, an antiscalant is selected and from the empirical data the program calculates the maximum recovery that can be achieved and what are the limiting salts. You can adjust the feed pH to see the impact of pH correction and you can also change some feed parameters so see how pretreatment will impact recovery. At this point, stages do not even come into the equation. For a customer that wants to get as high a recovery as possible, I also often ask the antiscalant suppliers to do a projection for me, because sometimes they have a product I may not be aware of or that is not part of the projection software version I have.

PLEASE NOTE - INCREASING THE NUMBER OF STAGES DOES NOT ALLOW A HIGHER RECOVERY THAN WHAT THE ANTISCALANT PROJECTIONS PREDICT!!! ALSO, INCREASING THE NUMBER OF STAGES DOES NOT ALLOW A HIGHER RECOVERY THAN WHAT THE ANTISCALANT PROJECTIONS PREDICT!!! Got the message??

Some antiscalant projection programs incorporate calculations on RO unit arrays, but I prefer to first optimize recovery then use the membrane manufacturer’s projection programs to design the best array – I just feel more confident with the RO/NF system design when I use the membrane vendor programs.

Now that I know the maximum recovery that is possible determined by the water chemistry, not the number of RO unit stages, I then go about working out how many stages are needed to achieve this recovery. When designing an array for an RO system there are several important design conditions that need to be accounted for, including crossflow velocity and flux across the membrane elements. The projection programs have built in warnings when these are too high or low. Operating outside the safe ranges will result in membrane fouling.

A 2-stage RO system is limited to up to 80-85% recovery because somewhere in this range you will violate the minimum crossflow requirements for the membranes. When the crossflow is too low you can get concentration polarization at the membrane surface resulting in scaling. By adding a third stage you can reduce the number of housings required in the first 2 stages to increase the crossflow velocity to within the desired range and add back those housings via the third stage to maintain the desired flux rate. For instance, rather than try achieving 85% recovery using a 4:2, 7M array at an average flux of 15 gfd at a permeate flow of 175 gpm which sends some low crossflow warnings for the first stage, you could change this array to a 3:2:1, 7M 3-stage system which has the same flux but the crossflows are better balanced across the membranes.

When attempting really high recoveries, where the water chemistry allows, you may even need to add a fourth stage. Another option to adding stages, is to recycle some Stage 2 concentrate back to the Stage 1 feed to increase cross flow velocity and allow higher recoveries but this increases the concentration of feed water constituents and subsequently the permeate quality is worse.

 And CCRO is not the silver bullet either!

Now a short discussion on ClosedCircuit RO (CCRO). The recovery rate of a CCRO system, like conventional RO, is limited by the chemistry and how much the feed water salts can be concentrated with antiscalant addition. In most cases, the recovery of CCRO is predicted by the antiscalant projections exactly the same as for conventional RO. Because CCRO purges the recycled concentrate every 15-45 minutes, if silica is the constituent limiting the recovery, due to the saturated silica concentration being reached towards the end of this cycle and the slower induction time for silica to precipitate once it reaches saturation (say 10 minutes), CCRO can push past the maximum recovery predicted by antiscalant projections. For that scenario, CCRO will achieve a higher recovery than a conventional RO system.

But for more common scalants such as calcium carbonate and calcium sulfate, the induction time is just seconds and the CCRO process offers no advantage over conventional RO if these are the limiting salts. I would be happy for someone to give me a good technical explanation how CCRO can achieve a higher recovery when CaCO3 and CaSO4 are the limiting salts because from my investigations I have not found one. Don't get me wrong, I'm all for new technologies that can improve on the recovery of conventional RO systems, but I also want to see good science on how this is achieved.

The main message I want you to get from this post though is the following:

INCREASING THE NUMBER OF STAGES DOES NOT ALLOW A HIGHER RECOVERY THAN WHAT THE ANTISCALANT PROJECTIONS PREDICT!!!

The comments and opinions in this post are my own and not those of my employer.